Boundary Waters algae blooms spark questions, concern

A person collects a sample in a lake
Lienne Sethna collects an algae sample on a lake in the Boundary Waters Canoe Area Wilderness late last month.
Courtesy of Greg Seitz

Adam Heathcote has been studying blue-green algae blooms for more than 15 years, including a lot of work in lakes in Iowa and southern Minnesota surrounded by farms and other development, where it’s common to see severe algae blooms.

But that didn't prepare him for what he saw last month in the Boundary Waters Canoe Area Wilderness, when he paddled into Burnt and Smoke Lakes — two small lakes off Sawbill Lake, a popular BWCA entry point north of Tofte, Minn.

A bright algae bloom along a lake shore
A neon blue algae bloom is seen on Burnt Lake in the Boundary Waters Canoe Area Wilderness last month.
Courtesy of Lienne Sethna

"That was as bad of a bloom as I saw in Iowa during my entire PhD, in a lake that was two portages into the Boundary Waters,” said Heathcote, who directs the Department of Water and Climate Change at the St. Croix Watershed Research Station, part of the Science Museum of Minnesota.

"I had never seen a bloom to that extent where it was like thick, neon-blue, neon-green paint, lake-wide, not just [in] a little isolated bay,” described Heathcote. “Two of the three lakes that we sampled in the Boundary Waters had this going on across pretty much their entire surface area."

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Heathcote was taken aback because these lakes are protected; they're surrounded by wilderness.

Algae exist in every lake. They are a healthy part of the ecosystem. It becomes problematic when the algae become dominated by cyanobacteria, which can produce toxins harmful to people and pets.

Not all algae blooms are toxic. But Lienne Sethna, a postdoctoral fellow working with Heathcote, said they looked at the algae found in the Boundary Waters under a microscope. And they spotted three species of cyanobacteria that produce different kinds of toxins.

“It’s something that we're really concerned about, and something we really hope to address through this study,” Sethna said.

Minnesota’s most pristine lakes

The researchers are studying some of the state’s cleanest, most pristine lakes, both within and just outside the wilderness area. They’re trying to figure out why the algae situation is unexpectedly changing. They also documented cyanobacteria in Elbow and Finger Lakes, part of the Timber-Frear canoe route in the Superior National Forest and on Sawbill Lake.

A paddle sits in lake water next to neon blue algae
A neon blue algae bloom is seen on Burnt Lake in the Boundary Waters Canoe Area Wilderness last month.
Courtesy of Adam Heathcote

The scientists are measuring nutrients and algae, water temperature and oxygen levels. They focused in part on Burnt and Smoke Lakes because of past reports of algae blooms there.

Clare Shirley owns Sawbill Canoe Outfitters with her husband Dan. She said her family remembers seeing algae on those lakes ever since her grandparents started the business in 1957.

Shirley said those lakes are shallow, with muddy bottoms and historically good walleye fishing that doesn’t seem to have been impacted by the algae.

“Nobody's ever gotten sick or reported issues that way either,” Shirley said. “So it's something we've kind of lived with for a very long time and always been aware of. But we're excited and grateful that somebody's studying it. And we'll be really curious to hear what they find out."

History in sediment

Heathcote said there’s evidence of algae blooms in shallow northern Minnesota lakes as far back as 18th century voyageur accounts. But he suspects that blooms in the area are becoming larger, more persistent and more toxic.

To verify that, scientists are extracting cores of sediment that gradually builds up on the lake bottoms. Cyanobacteria create pigments that are preserved in the sediment.

A person on a boat pulls up a sediment sample from a lake
Adam Heathcote pulls up a sediment core from East Twin Lake in the Superior National Forest.
Courtesy of Lienne Sethna

"And we can look back through those layers, almost like reading the pages of a book,” Sethna explained.

“We use those layers to understand how nutrients and algae have changed through time. And so we're using that to understand how cyanobacteria or harmful algal blooms have occurred in the past and what kinds of factors have changed from historic conditions to present day to help us understand why they're happening now."

Two possible causes

Scientists have two hypotheses for what's triggering blooms in these wilderness lakes.

One, climate change. Lakes are warmer. That allows nutrients that have accumulated in the sediment to be more readily available to algae at the top of the lake.

The second theory: dust. Researchers suspect nutrients are falling into lakes from the atmosphere and helping to fuel these algae blooms.

"We kind of had to pass the laugh test on that one first because of course, you think, ‘What could dust possibly do to these huge lakes?’" Heathcote admitted.

But he said several influential studies have documented the impact of dust in remote, high alpine lakes in the Rocky Mountain foothills in Colorado.

An algae bloom along a lake's shoreline
A bright green algae bloom is seen along the shoreline of Smoke Lake in the Boundary Waters.
Courtesy of Lienne Sethna

For the first time in Minnesota, Heathcote and his colleagues have set up a network of dry deposition monitors to capture and measure the nutrients that fall out of the sky.

Heathcote believes it’s likely that both global warming and atmospheric deposition are contributing to the increased trend of toxic cyanobacteria blooms in northern Minnesota. This project seeks to confirm that.

While a lot of research has been done on algae blooms in areas with obvious human impact, scientists are still working to understand the more global drivers that are fueling them.

“We don't actually know what we're going to find out,” Heathcote said. “This is truly cutting-edge research. This is a first-of-its-kind study in Minnesota and one of the first of its kind in the world.”

Anyone who sees what they suspect may be a harmful algae bloom on a remote northern Minnesota lake is asked to contact the researchers at or on twitter @scwrs_mn